ABSTRACT Avalonia and Ganderia are composite microcontinental fragments in the northern Appalachian orogen likely derived from Gondwanan sources. Avalonia includes numerous Neoproterozoic magmatic arc sequences that represent protracted and episodic subduction-related magmatism before deposition of an Ediacaran–Ordovician cover sequence of mainly siliciclastic rocks. We characterized the nature of the basement on which these arcs were constructed using zircon grains from arc-related magmatic rocks in Atlantic Canada that were analyzed for their Lu-Hf isotope composition. The majority of zircon grains from Avalonia are characterized by initial 176 Hf/ 177 Hf values that are more radiogenic than chondritic uniform reservoir, and calculated crust formation Hf T DM (i.e., depleted mantle) model ages range from 1.2 to 0.8 Ga. These data contrast with those from Ganderia, which show typically positive initial εHf values and Hf T DM model ages that imply magmatism was derived by melting of crustal sources with diverse ages ranging from ca. 1.8 to 1.0 Ga. The positive distribution of initial εHf values along with the pattern of Hf T DM model ages provide a clear record of two distinct subduction systems. Cryogenian–Ediacaran magmatism is interpreted to have resulted from reworking of an evolved Mesoproterozoic crustal component in a long-lived, subduction-dominated accretionary margin along the margin of northern Amazonia. A change in Hf isotope trajectory during the Ediacaran implies a greater contribution of isotopically evolved material consistent with an arc-arc–style collision of Ganderia with Avalonia. The shallow-sloping Hf isotopic pattern for Paleozoic Ganderian magmatism remains continuous for ~200 m.y., consistent with tectonic models of subduction in the Iapetus and Rheic Oceans and episodic accretion of juvenile crustal terranes to Laurentia.

Abstract The Neoproterozoic tectonomagmatic evolution of West Avalonia comprises four major events. Tectonism started with the formation of a Tonian passive margin on a Baltica-derived ribbon dispersed into the Mirovoi Ocean. Obduction of an oceanic terrane onto the ribbon produced olistostromes, deformation and metamorphism before 750 Ma. Obduction was followed by a Tonian (750–730 Ma) arc on the created composite crust. A pause in magmatism between 730 and 700 Ma is the next event. Subsequently, a Cyrogenian (700–670 Ma) arc was formed, which may have collided with Baltica or another buoyant element nearby. Thereafter, a long-lasting (640–565 Ma) continental arc was erected which, combined with the late Ediacaran–Early Paleozoic sedimentary cover, represents the hallmark of West Avalonia. A Caribbean-style incursion of the Ediacaran arc into the widening Tornquist gap between Amazonia and Baltica led to a diachronous collision with the Ganderian arc. Strike-slip slivering produced a complex transfer of terranes to both: Carolinia and smaller terranes to Ganderia, and East Avalonia to West Avalonia. The Rheic Ocean opened diachronously at c. 500 Ma, following a plate reorganization and re-establishment of an oblique subduction zone beneath Amazonia. As a result, Avalonia and Ganderia became progressively separated and dispersed into the Iapetus Ocean.

The evolution of many modern intra-oceanic and continental arc systems is exemplified by cycles of arc construction, rifting, and separation of remnant and active arcs by a backarc basin floored by oceanic crust. Rifted arc complexes and backarc basins are inherently subductable, and hence only a fragmentary record of rifting and arc construction is preserved in the ancient record. In this contribution, we synthesize available geochronological, geochemical, isotopic, and stratigraphic data in order to discuss the evolution of the Cambrian to Ordovician Penobscot-Victoria Arc, which developed on the leading edge of Ganderia, a peri-Gondwanan microcontinent. Although the Penobscot and Victoria stages of arc-backarc development occurred in a predominantly extensional suprasubduction-zone setting, they each display a distinctly different character of magmatism and sedimentation. These stages are separated by an orogenic episode marked by the obduction of backarc ophiolites onto the Ganderian passive margin. The Cambrian to Lower Ordovician Penobscot Arc is characterized by the continuous migration of the magmatic front and development of multiple volcanically active rift zones. The rift basins display a variety of characteristics, including bimodal calc-alkaline magmatism, felsic-dominated incipient rift magmatism, and formation of rifts floored by tholeiitic to boninitic suprasubduction-zone ophiolite. Comparison to modern analogues suggests that part of the Penobscot Arc area developed in a similar setting to the volcanically active Havre Trough and Taupo volcanic zone. In contrast, the Victoria Arc phase was dominated by multiple epiclastic rich, volcano-sedimentary basins overlying tectonically modified Penobscot basement. Igneous rocks are sparse, typified by calc-alkaline felsic volcanic and tholeiitic to alkaline backarc basin basalts. The change in character of the backarc volcanic rocks over time is interpreted to reflect multiple tectonic factors, including the variation of slab retreat rate, degree of extension in the arc (Cambrian Penobscot Arc) versus the backarc basin (Ordovician Exploits-Tetagouche backarc), reactivation of inverted Penobscot extensional faults during Middle Ordovician rifting, and/or depletion of fertile components by the Middle Ordovician.

The eastern flank of the Appalachian orogen is composed of extensive Neoproterozoic–early Paleozoic crustal blocks that originated in a peri-Gondwanan setting. Three of these blocks record the evolution of Neoproterozoic magmatic-arc systems, including Carolinia in the southern Appalachians and Ganderia and Avalonia in the northern Appalachians. Relationships among these three crustal blocks are important for understanding both the accretionary history of the orogen and the evolution of the Iapetus and Rheic Oceans, first-order geographic features of the Paleozoic globe. Traditionally, Carolinia and Avalonia have been considered to represent a single microcontinental magmatic arc that accreted to Laurentia in the middle to late Paleozoic. The early lithotectonic history (ca. 680–570 Ma) of the two blocks is obscure; however, their latest Neoproterozoic-Paleozoic histories are distinct. This disparity is manifest in the first-order features of (1) timing and style of magmatic-arc cessation and (2) the nature of their Paleozoic lithotectonic records. Magmatic arc activity ceased in Avalonia in the late Neoproterozoic (ca. 570 Ma), succeeded by extension-related magmatism and sedimentation that was transitional into a robust latest Neoproterozoic–Silurian platformal clastic sedimentary sequence. This platform was tectonically unperturbed until the Late Silurian–Early Devonian. In contrast, Carolinia records late Neo-proterozoic tectonothermal events coeval with arc magmatism, which extended into the Cambrian; a relatively thin Middle Cambrian shallow-marine clastic sequence is preserved unconformably atop the Carolinia arc sequences. Subsequently, Carolinia experienced widespread Late Ordovician–Silurian deformation and metamorphism. However, we note striking similarities between Carolinia and Ganderia; specifically, in Ganderia, like Carolinia, late Neoproterozoic tectonism was accompanied by arc magmatism that extended into the Cambrian. Ganderian arc rocks are capped unconformably by a Middle Cambrian to Early Ordovician clastic sequence, and they were tectonized in the Late Ordovician–Silurian, similar to relations in Carolinia. Independent studies indicate that the Late Ordovician–Silurian tectonism in both blocks was related to their accretion to Laurentia. Thus, Carolinia and Ganderia show parallel development of first-order lithotectonic characteristics for two endpoints in their global strain path, i.e., their Gondwanan source region and their accretion to Laurentia. Consequently, we posit that Carolinia appears to be more closely affiliated with Ganderia than with Avalonia. The recognition of this linkage between Appalachian peri-Gondwanan realm crustal blocks in light of paleomagnetic and isotopic data leads to a unified model for the accretion of these blocks to the eastern margin of Laurentia.

ABSTRACT Voluminous bimodal volcanic rocks of the Silurian (ca. 422–420 Ma) Dickie Cove Group in the Ganderia domain of northern New Brunswick, Canada, are subaerial units that were deposited in an extensional setting, with the mafic types corresponding to continental tholeiites. Felsic rocks are rhyolites with calc-alkaline affinities. They exhibit geochemical characteristics that are typical of A2-type felsic magmas, such as enrichments in the incompatible elements Zr, Nb, and Y, as well as high FeO*/(FeO* + MgO) and Ga/Al ratios. Their ε Nd ( t ) values are positive (+0.7 to +3.4) but lower than those of the associated basalts. Saturation thermometry has yielded average zircon crystallization temperature estimates for the rhyolites that are well above 900 °C. The geochemical data indicate that the felsic melts were likely sourced from heterogeneous Neoproterozoic lower crust and generated by dehydration melting triggered by heat derived from underplated mafic magma. Parent melts of the rhyolites underwent fractional crystallization in a complex magma chamber prior to eruption. The Nd isotopic data suggest that the lower crust of Ganderia is similar to that of Avalonia in northern mainland Nova Scotia, and that the two microcontinents shared a common Neoproterozoic history and origin as continental blocks rifted from neighboring parts of Gondwana. The tectono-magmatic setting of the Dickie Cove Group volcanic rocks is interpreted as being related to Pridolian, post-Salinic relaxation and slab breakoff, which generated volcanism initially constrained within the Chaleur zone of the Chaleur Bay synclinorium, a large domain of the northern Appalachians. This was followed later in the Pridolian by extensional collapse and widening of the area of magmatic activity, which then prograded into the Tobique zone farther to the southwest.

Abstract Uppermost Silurian–Lower Devonian felsic rocks in the bimodal volcanic suite of the Tobique Group in the northwestern mainland Appalachians (northern New Brunswick, Canada) form part of an overstep sequence deposited across the accreted vestiges of Iapetus Ocean on composite Laurentia. Whereas the mafic rocks of the bimodal volcanic suite are continental tholeiites, the felsic rocks are peraluminous and possess geochemical characteristics of A2-type granites emplaced in post-collisional extensional settings. The major and trace element compositions of the felsic rocks indicate that they were generated by dehydration melting of late Precambrian granitoid rocks triggered by heat derived from the rising mafic magma. Unlike the basalts, which have positive ε Nd ( t ) values, the felsic rocks have values close to chondrites (−1.6 to +1.1), which is consistent with derivation from a crustal source. The rapid transition from compressional to extensional magmatism in latest Silurian–Early Devonian times in this part of Ganderia is probably due to Late Silurian Ganderia–Laurentia collision followed by slab breakoff. Based on Sm–Nd isotopic characteristics in their respective igneous rocks, both Ganderia and Avalonia are underlain by similar Neoproterozoic lower–middle crust and subcontinental lithospheric mantle.

Abstract During the Early to Middle Palaeozoic, prior to formation of Pangaea, the Canadian and adjacent New England Appalachians evolved as an accretionary orogen. Episodic orogenesis mainly resulted from accretion of four microcontinents or crustal ribbons: Dashwoods, Ganderia, Avalonia and Meguma. Dashwoods is peri-Laurentian, whereas Ganderia, Avalonia and Meguma have Gondwanan provenance. Accretion led to a progressive eastwards (present co-ordinates) migration of the onset of collision-related deformation, metamorphism and magmatism. Voluminous, syn-collisional felsic granitoid-dominated pulses are explained as products of slab-breakoff rather than contemporaneous slab subduction. The four phases of orogenesis associated with accretion of these microcontinents are known as the Taconic, Salinic, Acadian and Neoacadian orogenies, respectively. The Ordovician Taconic orogeny was a composite event comprising three different phases, due to involvement of three peri-Laurentian oceanic and continental terranes. The Taconic orogeny was terminated with an arc–arc collision due to the docking of the active leading edge of Ganderia, the Popelogan–Victoria arc, to an active Laurentian margin (Red Indian Lake arc) during the Late Ordovician (460–450 Ma). The Salinic orogeny was due to Late Ordovician–Early Silurian (450–423 Ma) closure of the Tetagouche–Exploits backarc basin, which separated the active leading edge of Ganderia from its trailing passive edge, the Gander margin. Salinic closure was initiated following accretion of the active leading edge of Ganderia to Laurentia and stepping back of the west-directed subduction zone behind the accreted Popelogan–Victoria arc. The Salinic orogeny was immediately followed by Late Silurian–Early Devonian accretion of Avalonia (421–400 Ma) and Middle Devonian–Early Carboniferous accretion of Meguma (395–350 Ma), which led to the Acadian and Neoacadian orogenies, respectively. Each accretion took place after stepping-back of the west-dipping subduction zone behind an earlier accreted crustal ribbon, which led to progressive outboard growth of Laurentia. The Acadian orogeny was characterized by a flat-slab setting after the onset of collision, which coincided with rapid southerly palaeolatitudinal motion of Laurentia. Acadian orogenesis preferentially started in the hot and hence, weak backarc region. Subsequently it was characterized by a time-transgressive, hinterland migrating fold-and-thrust belt antithetic to the west-dipping A–subduction zone. The Acadian deformation front appears to have been closely tracked in space by migration of the Acadian magmatic front. Syn-orogenic, Acadian magmatism is interpreted to mainly represent partial melting of subducted fore-arc material and pockets of fluid-fluxed asthenosphere above the flat-slab, in areas where Ganderian's lithosphere was thinned by extension during Silurian subduction of the Acadian oceanic slab. Final Acadian magmatism from 395– c . 375 Ma is tentatively attributed to slab-breakoff. Neoacadian accretion of Meguma was accommodated by wedging of the leading edge of Laurentia, which at this time was represented by Avalonia. The Neoacadian was devoid of any accompanying arc magmatism, probably because it was characterized by a flat-slab setting throughout its history.